The RGB (Additive) Colour mode controls the aspect of Red Green and Blue amounts in the foreground or background colour of the chosen part you want coloured. Having 255 of a certain colour will make it the fullest version of the colour you choose to make it, either Red Green or Blue, though you must also make the other colours, 0 like in this example shown. Alternatively making the colours different numbers all having a value will mix the colours together though, creating a combination of the colours. Essentially this colour mode brightens the colours used and chosen. RGB is used for computer displays.
Moving on to the CMYK (Subtractive) Colour mode this mode C stands for Cyan, M Magenta, Y for Yellow and K for Key – the key colour these days being black. And in turn controls the amount of these set colours being distributed into the colour area you want to change. Altering the colour, differently than RGB, making it less intense in a sense, more darker. CMYK is used for hard copies of printed colour illustrations.
In the bottom area under HSB and RGB the input area with the # key in front of it is where hexadecimal keys are used for colour picking and defines the colour you have in the screen area. It refers to screen colours.
HSB is Hue Saturation and Brightness, and controls these aspects of the colour you are working with. Changing the colour with hue, and the intensity of the colour as well. Brightness clearly controls how bright and I suppose intense that the colour is like saturation too.
Changing all of this, making a picture or a piece of artwork greyscale removes the colour completely from it.
In pulling up a histogram panel in photoshop we are shown the tonal range – the key type of an image by showing how the pixels are put around with a graph of the number of pixels that are at each of the 256 brightness levels in the image.
An image will have a transparent layer if the background is checkerboard and to retain and keep the transparency we need to save the file as a .PNG. Select and mask can also be used to make something inherently transparent, showing the checkerboard behind it. Like with the background I was using that was white in this one.
Alpha channels get their name from something called alpha compositing, combining a partially transparent image with another image. the information about the shape of the transparent area is kept in the alpha channel safe and sound. It’s saving your selections for you to go back on and change if you feel like it.
Heather Barbeau's Blog 